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Science of the Total Environment 685 (2019) 10–18

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Alkali resistant nanocomposite gel beads as renewable adsorbents forwater phosphate recovery

Xuanqi Huang, Wufeng Wu, Yan Xia, Wanbin Li, Yanyan Gong, Zhanjun Li ⁎Guangdong Key Laboratory of Environmental Pollution and Health, School of Environment, Jinan University, Guangzhou 510632, China

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• Hydrous zirconium oxide/alginatebeads (ZrA) are prepared for phospho-rus recovery.

• Sr2 is the only crosslinking ion that canproduce alkali resistant ZrA beads.

• ZrA is mesoporous (15.3 nm) with aspecific surface area of 80.84 m2·g−1.

• ZrA shows a high Langmuir adsorptioncapacity to phosphate (52.5 mg-P·g−1).

• ZrA can be easily recycled and protectedfrom adverse humic acid contamination.

⁎ Corresponding author.E-mail address: [email protected] (Z. Li).

https://doi.org/10.1016/j.scitotenv.2019.05.2960048-9697/© 2019 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 15 March 2019Received in revised form 17 May 2019Accepted 20 May 2019Available online 21 May 2019

Editor: Huu Hao Ngo

Hydrous zirconium oxide (HZO) encapsulated alginate gel beads were synthesized for phosphate recovery fromwater. Importantly, we find that HZO/alginate gel beads (ZrA) crosslinked with Ca2 , Mg2 , Fe3 , Al3 , and Zr4 areunstable under an intense alkali regeneration condition. Only Sr2 -crosslinked ZrA can endure a high alkali solu-tion. ZrA possesses a high specific surface area (80.84 m2·g−1) and a mesoporous structure (15.3 nm and0.196 cm3·g−1), which endow them with a high Langmuir adsorption capacity of 52.5 mg-P/g. ZrA can be easilyrecycled, and themass loss of HZO is prevented. Furthermore, the strontium alginate gel framework protects theencapsulated HZO nanoparticles from adverse humic acid contamination. ZrA can be regenerated for at least 5adsorption/desorption cycles. Cost analysis indicates the potential scale application feasibility for ZrA. Thisstudy provides a novel, simple, and environmentally benign solution to immobilize HZO for efficient phosphaterecovery.

© 2019 Elsevier B.V. All rights reserved.

Keywords:Phosphate recoveryGel beadsAlginateHydrous zirconium oxideNanomaterials

1. Introduction

Phosphorus pollution is among the most important contributors toglobal water eutrophication, which threatens global aquatic ecosystemsand can lead to dramatic economic loss and even damage to human

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11X. Huang et al. / Science of the Total Environment 685 (2019) 10–18

health via cyanotoxin production (Mayer et al., 2016). However, phos-phorus is a basic element in all organisms and of critical importance inmodern agriculture (Tarayre et al., 2016). However, phosphate ore is anon-renewable resource that will expire in the foreseeable future.Thus, phosphorus recovery from wastewater is not only necessary foraquatic environment protection but also for human sustainable devel-opment (Desmidt et al., 2015).

Considerable efforts have been made regarding phosphate removalor recycling from wastewater, for example, the chemical precipitationmethod of adding calcium or magnesium-rich reagents or materials(Ye et al., 2017). However, most of the existing technologies onlywork well at high phosphate concentrations. There are still challengesin recyclingphosphate at a low concentration. Adsorption is a promisingtechnology for phosphate recovery, as it is easy to handle and cost-effective (Loganathan et al., 2014). Many adsorbents have been devel-oped with a high adsorption capacity; most of these adsorbents arenanomaterials that have a high specific surface area, such as hydrouszirconium oxide nanoparticles (HZOs) (Awual et al., 2011; Chitrakaret al., 2006), iron oxide (Kumar et al., 2017; Othman et al., 2018), MgO(Li et al., 2017; Zhu et al., 2018), lanthanum oxide (Dong et al., 2017;Fang et al., 2017), layered double hydroxides (Everaert et al., 2016),and their composites (Chen et al., 2015; Du et al., 2019; Fang et al.,2018; Qiu et al., 2015; Zhou et al., 2018). HZO is among the best phos-phorus recoverymaterials not only because it possesses a very high ad-sorption capacity but also because it can be easily regenerated viaalkaline washing (Awual et al., 2011; Chitrakar et al., 2006). Althoughthe excellent phosphate removal property of HZO has been proven, sev-eral problems remain that hinder its practical applications. First, powderadsorbents are inappropriate for large-scale applications because of thedifficulty in separating them during the post-treatment step. It is evenmore difficult to recycle HZO nanoparticles from water because oftheir Brownianmovement and electrostatic repulsion. Second, the irre-versible adsorption of chelating macromolecules (co-existing undernatural conditions, such as humic acid) may lead to a dramatic decreasein adsorption capacity. To solve these problems, a number of strategieshave been proposed, including loading HZO within porous bulk mate-rials. HZO has been encapsulated in a porous ionic exchange resin andshowed outstanding properties for adsorption removal of phosphate,fluoride, arsenic, and so on (Chen et al., 2015; Pan et al., 2014; Zhanget al., 2017). However, a synthetic ion exchange resin is a type of petro-chemical that is relatively expensive. A more environmentally benignand cost-effectivemethod is still needed to promote possibleHZO appli-cations in phosphorus recovery.

Alginate is a naturally occurring polysaccharide derived from the al-kaline digestion of algae or bacteria (Shroff et al., 2018). Alginate caneasily form ionic crosslinked alginate gel beads (ALGs) in the presenceof multivalent metal ions, such as Ca2 , Sr2 , Fe2 , Fe3 , Al3 , and Zr4

(Fernando et al., 2019). Functional nanoparticles are encapsulated intoALG for diverse environmental applications (Fernando et al., 2019).Meenakshi et al. synthesized ALG crosslinked by Zr(IV) in waterdefluoridation (Prabhu and Meenakshi, 2015). Jeon and co-workersimmobilized zirconium oxide within ALG and successfully utilized itfor efficient adsorption removal of As(III), As(V), Cu(II), and Cr(VI)fromwater (Kumar et al., 2018; Kwon et al., 2016). Lee's groupdesignedCa2 -crosslinked ALG to encapsulate nano-AlOOH modified biochar forwastewater phosphate recovery applications (Jung et al., 2017). Al-though many studies have been performed, little effort has been madein phosphate recovery using HZO-encapsulated ALG. The potential in-fluence of ALG on the performance of HZO remains unclear. Further-more, limited attention has been paid to the regeneration ofnanoparticle-encapsulated ALG adsorbents, possibly because of thepoor stability of Ca2 -crosslinked ALG under an intense alkali condition,which is needed for adsorbed phosphate desorption.

Inspired by the aforementioned works regarding HZO and ALG, weproposed to encapsulate HZO into ALG beads to produce environmen-tally benign, alkali resistant and renewable HZO-loaded ALG beads for

phosphorus recovery. SrCl2, CaCl2, MgCl2, FeCl3, AlCl3, and ZrOCl2 wereused as diverse crosslinking reagents to prepare gel beads, while Sr2 -crosslinked HZO-ALG gel beads (ZrA) were the only stable beadsunder an intense alkali condition. HZO was immobilized within theALG hydrogel framework, such that HZO could be easily separatedfrom the treated water while avoiding nano-adsorbent mass loss withthe effluent. Furthermore, HZO was shielded from the possible adverseinfluence of co-existing macromolecules, such as humic acid (HA)(Fig. 1). The influence of the ALG gel on the HZO adsorption propertieswas studied by determining the ZrA adsorption isotherms and kineticsin comparison to those of HZO. A regeneration experiment was con-ducted to explore the feasibility of reusing ZrA for multiple phosphaterecovery cycles.

2. Experimental

2.1. Materials

Zirconium oxychloride was used as the Zr(IV) source for the HZO.Chemical pure sodium alginate (viscosity, 200 mPa·s) was purchasedfrom Macklin (Shanghai, China). SrCl2 was used as a crosslinking re-agent for sodium alginate gelation. Phosphate solution was preparedby dissolvingNaH2PO4. Its concentrationwas calculated by its phospho-rousmass content. NaOH andHCl concentrated acidwere used to adjustthe pH of the solutions. Ammonium molybdate, antimony potassiumtartrate, and ascorbic acid were used in phosphate concentration test-ing. All the chemicals were A.R. grade unless otherwise stated.

2.2. Synthesis of hydrous zirconium oxide (HZO)

HZOwas synthesized using a simple precipitation method. Briefly, aZrOCl2 aqueous solution (0.1 M) was prepared. NaOH solution (1 M)was added in a drop-by-dropmanner to obtain a pH thatwasweakly al-kaline (pH = 7–8) under vigorous stirring. The mixture was furtherstirred for 30 min. After ceasing the stirring, the HZO was separatedvia centrifugation. The supernatant was discarded. Water was addedto wash the HZO precipitate to remove additional salt and base, if any.HZO colloidal dispersion (0.1 M, calculated by Zr) was finally obtainedby adding water. Its mass concentration was 16.02 g/L tested byweighing the dried HZO powder of a certain volume of HZO colloid.Real-time stirring was performed when HZO dispersion was used forHZO/ALG gel bead synthesis. An HZO powder sample was also preparedvia centrifugation and oven drying at 105 °C for 24 h.

2.3. Synthesis of the ZrA beads

Sodium alginate solution (2 wt%) was prepared by dissolving so-dium alginate powder into water. Heat was used to facilitate dissolu-tion. The HZO colloidal dispersion and ALG solution were mixed bystirring at a volume ratio of 6:4. SrCl2·6H2O solution (2 wt%) was pre-pared as the crosslinking bath solution. A total of 200 mL of the HZO/ALG mixture was dropwise added to 1000 mL of crosslinking baththrough a pipette tip at a rate of 2mL/min. Gel beads were immediatelyformedwhen the dropletswere immersed into the crosslinking bath so-lution. Stirring for 1 h was used to complete the gelation reaction. Thegel beads were filtered and washed with water three times. The ob-tained ZrA beads were stored in water at 4 °C. Some ZrA beads werefreeze dried and then oven dried at 105 °C for further characterization.

CaCl2, MgCl2, FeCl3, AlCl3, and ZrOCl2 were also used to prepare gelbeads according to the same procedure by replacing SrCl2.

2.4. Adsorption isotherms of phosphate

To accurately measure the mass of the HZO, dried HZO powder wasused for the isotherm. The as-synthesized ZrA beads or HZO (1 g/L)were added to 25 mL of phosphate aqueous solution with diverse

Fig. 1. Schematic illustration of the ZrA gel bead preparation and its application in phosphate recovery.

12 X. Huang et al. / Science of the Total Environment 685 (2019) 10–18

phosphorus concentrations and shaken for 24 h in a thermostatic oscil-lator. The phosphate recovery efficiency (η) and equilibrium adsorptioncapacity (qe, mg-P/g-adsorbent) were calculated using Eqs. (1) and (2),respectively, as follows:

η ¼ C0−Ceð ÞC0

� 100% ð1Þ

qe ¼C0−Ceð Þ � V

mð2Þ

where C0 (mg/L) is the initial phosphorus concentration, Ce (mg/L) isthe equilibrium phosphorus concentration, m (g) is the gel bead drymass, and V (L) is the phosphate contaminated water volume. The ad-sorption isotherms were fitted using the following equations:

Langmuir Eq. (3):

1qe

¼ 1qmklCe

þ 1qm

ð3Þ

Freundlich Eq. (4):

lnqe ¼1n

lnCe þ lnkf ð4Þ

where qe and qm (mg-P/g-adsorbent) are the equilibrium and max ad-sorption capacity, respectively; Ce (mg/L) is the equilibrium phosphateconcentration and kl and kf, n are constants.

2.5. Adsorption kinetics

A total of 0.1 g of ZrA beads (drymass) or HZO powderwas added to100mL of phosphate solution (10mg-P/L) and shaken in a thermostaticoscillator. A 0.2mL aliquot of the solutionwas taken for a phosphate testat predetermined time intervals. The equilibrium adsorption capacity(qe) was measured after 24 h. The adsorption kinetics were fittedusing pseudo first-order (Eq. (5)) and second-order (Eq. (6)) modelsas follows:

ln qe−qtð Þ ¼ lnqe−k1 � t ð5Þ

tqt

¼ 1q2eK2

þ tqe

ð6Þ

where qt and qe (mg-P/g-adsorbent) are the real-time and equilibriumphosphorus adsorption capacities, respectively; t (h) is the adsorptiontime; and k1(h−1)and k2(g mg−1 h−1)are the rate constants of the firstand second fitting models, respectively.

Table 1Alkali resistance (NaOH, 5 wt%, 2 h) of ZrA beads using diverse crossing ions.

Crosslinking reagents SrCl2 CaCl2 MgCl2 FeCl3 AlCl3 ZrOCl2

Alkali resistance Stable Fragile Broken Broken Dissolved Broken


2.6. Influence of preloading of humic acid

A total of 0.1 g of HZOpowderwas dispersed in 500mLof humic acidsolution (0.1 g/L) and stirred for 24 h at room temperature. Humic acidpre-loaded HZO powder (HZO-HA) was obtained after centrifugationand oven dried at 105 °C for 24 h. A 0.5-g HZO control sample (HZO)was obtained using the same procedure without humic acid addition.The mass of the obtained HZO-HA and HZO was measured to calculatethe mass loss during the preloading process. HA pre-loaded gel beads(ZrA-HA) were obtained using the same method as that for HZO-HApreparation. The HZO, HZO-HA, ZrA, and ZrA-HA phosphate adsorptioncapacitywere tested using the samemethod as that for testing the influ-ence of coexisting ions.

2.7. Regeneration of phosphate-adsorbed gel beads

A total of 0.1 g of ZrA was added to 100 mL of phosphate solution(10 mg-P/L) and shaken for 24 h in a thermostatic oscillator. The phos-phorus concentration was tested to evaluate the removal efficiency.Phosphate-adsorbed ZrA was placed in 5 mL of NaOH solution (5 wt%)and shaken for 0.5 h in an oscillator at room temperature. After filtra-tion, the gel beads were washed using flowing water to remove the ex-cess NaOH. The residue base was removed via a regeneration solutioncomposed of 0.05mol/L SrCl2 and 1mmol/L HCl. SrCl2 was used to com-pensate for the possible metal ion loss during the adsorption processwhile HCl was used to remove the residue NaOH. Flowing water wasagain used to wash the gel beads to eliminate the residual Sr2 influenceon phosphate adsorption. Regenerated ZrA was finally obtained for thenext phosphorus recovery cycle. Five adsorption/desorption cycleswere performed to evaluate the ZrA regeneration property.

2.8. Characterizations

To prepare the dried samples for characterization, the gel beadswere freeze dried for 3 d and then oven dried at 105 °C for 24 h. Thewater content of the gel beads and the mean dry mass of the singlegel beads were measured and calculated by the mass of 100 wet anddry gel beads. The phosphate concentration (calculated as phosphoruselement) was determined using the standard molybdenum bluemethod (GB11893-89, China) on a UV/VIS spectrophotometer (UV-1780, SHIMADZU, China). The X-ray diffraction (XRD) pattern was ac-quired using a powder diffractometer with Cu Kα radiation (λ =1.5418 Å) (D2 PHASER, AXS, Germany). The dried beadmicrostructureswere observed using a field emission scanning electron microscope(FSEM) with an acceleration voltage of 5 kV (S-4800, Hitachi, Japan).The infrared absorption spectra were collected using a Fourier trans-form infrared (FTIR) spectrometer (IRTracer-100, Shimadzu, Japan).The hydrodynamic size distribution of the HZO nanoparticles was ac-quired using a dynamic light-scattering device (Zetasizer Nano ZSE,Malvin, UK).

3. Results and discussion

3.1. Alkali resistance of ZrA gel beads using diverse crosslinking ions

To reduce the economic cost of adsorbents, they should be reusable.Adsorption-saturated HZO regeneration needs to be conducted underan intense alkali condition to desorb phosphate from the HZO particlesurface. Thus, ZrA beads should be alkali resistant to ensure successfulregeneration such that they can be reused multiple times. We studiedthe influence of diverse crosslinking reagents on ZrA bead alkali resis-tance. Interestingly, we found that alginate calcium beads, which aremost frequently reported using CaCl2 to crosslink alginate, are fragileunder an alkali condition (NaOH, 1 M, 2 h) and will break into piecesafter slight shaking or stirring. We further explored several other multi-valent metal ions as potential crosslinking reagents (Table 1). The

results indicate that using MgCl2, FeCl3, and ZrOCl2 as crosslinking re-agents will generate alginate beads that are unstable under an alkalicondition and will be broken after immersion into an NaOH solutioneven without any shaking or stirring. Using AlCl3 will generate alginatealuminum beads that can be dissolved into an NaOH solution, possiblybecause of the amphoteric property of the aluminum ion. Only alginatestrontiumgel beads using SrCl2 as crosslinking reagents are stable underan alkali condition. Possible reactions and their corresponding activityproducts are listed in Eqs. ((7)–(12)) based on the assumption that1 mol of ALG‧nM gel was decomposed into 1 mol of alginate and nmol of metal ions (Mz ). The activity product (Ksp3) of the gel decompo-sition reaction (Eq. (12)) has a negative correlation with Ksp2, which isthe metal hydroxide solubility product constant. We compared the sol-ubility product constants (pKsp) of the correspondingmetal hydroxides.The results indicate that Sr(OH)2 has the smallest value (2.5), several or-ders of magnitude smaller than that of Ca(OH)2 (5.3), Mg(OH)2 (11.7),Fe(OH)3 (37.4), Al(OH)3 (32.3), and ZrO(OH)2 (48.2); thus, ALG-Sr istheoretically the most stable gel. Ca2 , Mg2 , Fe3 , and ZrO2 may losetheir crosslinking ability under an alkali condition by formingmore sta-ble metal hydroxides. Similar result was found in an existing reportshowing the advanced stability of ALG-Sr over ALG-Ca (Dechojarassriet al., 2018). The hereafter mentioned ZrA beads were prepared usingSrCl2.

alginate � nM ¼ alginatenz− þ nMzþ ð7Þ

Ksp1 ¼ alginatenz−� � � Mzþ� �n ð8Þ

M OHð Þz ¼ Mzþ þ zOH− ð9Þ

Ksp2 ¼ Mzþ� � � OH−½ �z ð10Þ

alginate � nMþ nzOH− ¼ alginatenz− þ nM OHð Þz ð11Þ

Ksp3 ¼ alginatenz−� �

= OH−½ �nz ¼ Ksp1= Ksp2� �n ð12Þ

3.2. Physical structures of the as-synthesized ZrA beads

HZOwas synthesized by simply precipitating zirconiumoxychlorideaqueous solution under an alkali condition (pH adjusted by addingNaOH solution) at a size of tens of nanometers (Fig. S1). ZrA gel beadsimmediately formed after the ALG/HZO colloidal dispersion was addeddropwise into the Sr2 solution. The as-synthesized ZrA beads have auniform spherical morphology with a narrow size distribution of 1.89± 0.16 mm (Fig. 2). The milky color is induced by the HZO color. TheHZO loading efficiency in ZrA is calculated according to the Zrmass con-servation mechanism and the tested dry mass of the obtained ZrA. TheoptimizedHZO content in ZrA is 44.3% under our experiment conditions(Fig. S2). The highHZO loading efficiency in ZrAwill ensure a high phos-phate recovery capacity. However, further increasing the HZO contentwill lead to weakening or even breakage of the as-prepared ZrA beads.The XRDpatterns of the ALG gel beads, HZO, and ZrA beads show no ob-vious diffraction characteristics, indicating that the as-synthesized HZOand theHZO encapsulatedwithin ZrA are both amorphous (Fig. S3). TheZrA FTIR absorption spectrum shows a new peak at ~851 cm−1, indicat-ing possible chemical bonding between the ALG carboxyl group and theHZO Zr atom (Fig. S4) (Prabhu and Meenakshi, 2015). This chemicalbonding may aid in immobilizing HZO nanoparticles within the gelframework.

Fig. 2. Optical image of ZrA gel beads.


The microscopic morphology of the ZrA dried gel beads was ob-served using SEM. The ZrA sample appears as a plump granular(Fig. 3a) and its surface is full of nanoparticles (Fig. 3b). After zoomingin on the surface, one can see that the surface contains numerous wrin-kles (ALG polymer) and agglomerated nanoparticles (HZO) (Fig. 3c andd). A cross section image was taken along the cutting section (Fig. 3eand f) inwhich one can see that the ZrA beads contain abundant porousspaces.

The ZrA porous characteristic was further confirmed by theHZO andZrA N2 adsorption/desorption isotherms. The HZO and ZrA specific sur-face areas are 223.2 m2·g−1 and 80.84 m2·g−1, respectively. The hyster-esis loops indicate a significantmesoporous character (Fig. 4a). Notably,

Fig. 3. SEM images of the surface (a–d) and c

the HZO mesopores may more likely be caused by the spaces betweenadjacent nanoparticles in the HZO dried powder sample. No mesoporesexist in the HZO that are larger than 5 nm in diameter (Fig. 4b). Regard-ing ZrA, mesopores exist within the whole testing range from ~1 to~24 nm with a mean pore size of 15.3 nm and a pore volume of0.196 cm3·g−1. The larger pore size is favored to ensure rapid masstransfer during adsorption.

EDX elemental mapping was performed on a cross section. Theresults shown in Fig. 5 indicate that ZrA contains abundant Zr andSr elements. Their distributions are generally very uniform withinthe gel beads at a micrometer scale, possibly benefitting from thestabilization effect of alginate molecules on the HZO distribution.The uniform HZO distribution within ZrA is helpful in maintainingits good adsorption capacity, as severe agglomeration may possiblylead to HZO Ostwald ripening of HZO and in turn a decreased adsorp-tion capacity.

3.3. Phosphate adsorption capacity of the ZrA beads

The ZrA adsorption capacity is evaluated by testing its adsorptionisotherm in comparison to that of HZO, which can be better fitted byusing the Langmuir model (solid lines) rather than the Freundlichmodel (dash lines) as shown in Fig. 6 and Table 2. The HZO Langmuirmaximum adsorption capacity (qm) can reach 109.4 mg-P/g. The ZrAqm is fitted at 52.5 mg-P/g, which is relatively higher not only thanother zirconium-based adsorbents, such as commercial zirconium hy-droxide (Johir et al., 2016), ZnFeZr nanoparticles (Schneider et al.,2017), Zr/Fe modified carbon fiber (Xiong et al., 2017), HZO-loadedD201 resin (Chen et al., 2015) and Zr/Fe modified 1402 resin (Zhouet al., 2018), but also higher than the reported alginate-encapsulatedAlOOH/biochar/ALG gel beads (Jung et al., 2017) and Al-bentonite/ALGbeads (Pawar et al., 2016), as shown in Table 3. The possible reasonfor the higher qm could be the high specific surface area(80.84 m2·g−1), mesoporous structure, and high HZO loading efficiencyin ZrA (44.3%, Fig. S2). Although Zr-modified membranes have a betterphosphate adsorption capacity, their synthesis is much more complexand relies on expensive membrane support (Furuya et al., 2017).

ross section (e, f) of dried ZrA gel beads.

Fig. 4. HZO and ZrA N2 adsorption/desorption isotherms (a) and pore size distributions (b).


3.4. Adsorption kinetics of ZrA

A kinetic studywas performed to evaluate the ZrA phosphate recov-ery speed in comparison to that of HZO. The results shown in Fig. S5 in-dicate that HZO can more quickly reach an adsorption equilibriumcompared to ZrA. Both the kinetics can be better fitted using apseudo-second order model (R2 N 0.99) than a first-order (Figs. S6 andS7). The adsorption kinetic constant of HZO (0.811) is much biggerthan that of ZrA (0.125), as shown in Table S1. The reason may be be-cause of the steric-hindrance effect of the ALG gel framework, whichgenerates additional mass transfer resistance. This problem may be re-lieved by using smaller ZrA beads.

Fig. 5. EDX elemental mapping of C, Sr, and

3.5. Advantages of ZrA beads in preventing HZO nanoparticle mass loss

There is a consensus that nanoparticles are difficult to recycle be-cause of theirwell-known Brownianmotion and electrostatic repulsion.The as-synthesized HZO nanoparticles have a very small size of tens ofnanometers (Fig. S1). Its recyclability will be a potential problem hin-dering its practical application. We dispersed HZO into water with vig-orous stirring at a concentration of 0.1 g/L. Gravity sedimentation wasused to recycle the adsorbents. After 2 h of natural sedimentation, inthe case of HZO, ~90% of the supernatant was carefully discardedwhile the remaining dispersion was oven dried to recycle the HZO.The results indicate that the recycled HZO mass quickly decreased

Zr in a cross section of ZrA gel beads.

Fig. 6. HZO and ZrA gel bead adsorption isotherms. The solid and dashed lines are theLangmuir and Freundlich fitted curves of the isotherms, respectively.

Table 2Langmuir and Freundlich fitting of the adsorption isotherms.

Sample Langmuir Freundlich

qm(mg/g)

kl(L mg−1)

R2 1/n kf(L mg−1)(L mg−1)1/n

R2

HZO 109.4 0.199 0.997 0.546 19.3 0.931ZrA 52.5 0.344 0.993 0.384 16.4 0.870

Fig. 7. Recycling of adsorbents.


after each cycle (Fig. 7). After five dispersion/recycle cycles, only ~43%could be recycled. The lost HZO will not only lead to a decrease in thephosphorus recycling efficiency and an increase in the cost of purchas-ing additional adsorbent butmay also lead to a new environmental con-cern as a nano-pollutant. In the case of ZrA (0.1 g/L, dry massconcentration), all the ZrA beads precipitated to the bottomwithin sec-onds. There were basically no technical difficulties in the separation ofthe millimeter particles from the water by discarding the supernatant.After five cycles, nearly 100% of the ZrA beads were successfullyrecycled.

3.6. Advantages of ZrA beads against humic acid contamination

Surface water contains many types of natural macromolecules rep-resented by humic acid (HA). HA is the residue of dead plants and ani-mal bodies after natural degradation that can stably exist in water andsoil for hundreds of years under natural conditions. HA is composed ofmany oxygen-containing groups (such as the carboxyl, aldehyde, hy-droxyl, and phenolic hydroxyl groups) and nitrogen-containing groups(such as the amino, imine, and azacyclo groups) that can form stable

Table 3Comparison of the phosphorus recovery capacities of various adsorbents.

Adsorbent pH qmmg/g

Ref.

Zr-oxide, Hydrothermal 6.2 99.0 (Su et al., 2013)Zr-hydroxide, commercial 7.1 18.5 (Johir et al., 2016)ZnFeZr nanoparticles 7 20.3 (Schneider et al., 2017)Zr/Fe modified carbon fiber 6.8 26.3 (Xiong et al., 2017)Zr modified membrane 5 64.8 (Furuya et al., 2017)HZO-loaded D201 resin 6.5 46.0 (Chen et al., 2015)Zr/Fe modified 1402 resin 7 37.9 (Zhou et al., 2018)AlOOH/biochar/ALG gel beads 6 48.8 (Jung et al., 2017)Al-bentonite/ALG gel beads 7.8 11 (Pawar et al., 2016)ZrA beads 7 52.5 This work

complexes with metal ions and hydroxides. Therefore, HA may beadsorbed on HZO and, as a competition, decrease its phosphate adsorp-tion capacity. We preloaded HA on HZO and ZrA bymixing these adsor-bents (200 mg/L) into an HA solution (100 mg/L). After 24 h of stirringat room temperature, HZO were recycled and washed by water 3 timesusing a high-speed centrifuge. The HA-loaded HZO (HZO-HA) has abrown color (the inset of Fig. 8), indicating obvious intense adsorptionof HA on HZO which can be further confirmed by the FTIR analysis(Fig. S8). Furthermore, in a comparative study with HZO, one can seethat the HZO-HA phosphorus recovery efficiency is obviously sup-pressed as its recovery efficiency decreased from 98.7% (HZO) to20.1%, as shown in Fig. 8.

The intense HA inhibition effect has to be considered for large-scaleHZO phosphorus recovery applications because HA extensively occursin natural aquatic environments. Regarding HA preloaded ZrA beads(ZrA-HA), only a weak influence can be observed as the recovery effi-ciency slightly decreased from 94.6 to 91.8%. The resistance of ZrA to

Fig. 8. Influence of humic acid (HA) contamination on phosphorus recovery. The inset is anoptical image of HA-contaminated HZO (C0 = 10 mg-P/L, CZrA = 1.0 g/L, and t = 24 h).

Fig. 9. Influence of the ZrA dosage on phosphorus recovery (C0 = 10 mg-P/L, CZrA =1.0 g/L, and t = 10 h).


HA contamination can be attributed to the steric-hindrance effect of theALG polymer framework against HA. HA is a mixture of natural poly-mers with molecular weights mainly ranging from several thousandsto hundreds of thousands. These macromolecules can be blocked fromreaching the inside of ZrA and affecting the adsorption capacity of theHZO encapsulated within the ALG gel framework.

3.7. Performance of ZrA beads in simulatedwastewater phosphate recovery

ZrA beads were used to treat artificial phosphate-containing waste-waterwith a low concentration of 10mg-P/L. The results shown in Fig. 9indicate that the phosphorus recovery efficiency quickly increased from25.6 to 86.9% along with the increase in the ZrA mass concentration(CZrA) from 0.1 to 0.6 g/L. However, the efficiency only slowly increasedafter further increasing CZrA. The optimal ZrA dosage is proposed to be~1.0 g/L as the residue phosphorus concentration is already lowerthan 0.5mg-P/L, which is Standard A of thefirst class in “Discharge stan-dard of pollutants for municipal wastewater treatment plant” of China(GB18918–2002).

3.8. Regeneration under an intense alkali condition

The adsorption capacity of ZrA is pH sensitive and can be inhibitedunder alkali conditions (Fig. S9). The phosphate-adsorbed ZrA was re-generated in 5 wt% NaOH. After five cycles of regeneration, ~80%

Fig. 10. Regeneration of ZrA gel beads (C0 = 10 mg-P/L, CZrA = 1.0 g/L, and t = 10 h).

phosphorus recovery efficiency remained, which indicates good reuseproperties that will dramatically decrease the material cost (Fig. 10).Even after overdue because of material aging, Zr and other solubleions can be easily recycled via acid dissolution while residue alginatewill precipitate as alginate acid which is a part of the alginate sodiumproduction and purification process. As alginate is a type of bio-polymer material and environmentally benign, the whole ZrA lifecycle is not likely to generate or release environmentally concerningpollutants. Regarding other adsorbents, such as an ionic exchange poly-meric resin, they are essentially petrochemical products (Chen et al.,2015; Zhou et al., 2018). Their usagemay possibly generate plastic frag-ments and particles after the end of their life cycle. As the size of theresin generally ranges from hundreds of micrometers to several milli-meters, it will likely generate environmentally concerning microplasticpollutants that are an emerging environmental concern as potentialnew threats to aquatic ecosystems (Arias-Andres et al., 2018; Coleet al., 2011).

3.9. Economic cost analysis of raw materials for ZrA beads

A simple cost analysis was performed according to the chemicalprices from Chemical Platform of the Chinese Academy of Sciences(CASMART). The results show that SrCl2 is more expensive than CaCl2(Table 4). However, the main cost in producing ZrA is the purchase ofZrOCl2 (80.7%) and alginate sodium (15%) while the cost of SrCl2 isonly 4.3% of the total cost. Using SrCl2 instead of CaCl2 will not lead toa significant increase in the total material cost. Moreover, using SrCl2as a crosslinking reagent is the only means to regenerate ZrA beadsunder an intense alkali condition (Table 1), which will significantly re-duce the potential running cost of adsorbents. Therefore, using SrCl2,rather than CaCl2, to produce ZrA beads will be feasible for potentialscale production.

4. Conclusion

Alkali resistant strontium alginate gel beads are successfully synthe-sized to immobilize HZO nanoparticles to produce ZrA gel beads forwastewater phosphate recovery. Sr2 -crosslinked ZrA beads can endurean intense alkali condition,while other common ions, such as Ca2 , Mg2 ,Fe3 , Al3 , and Zr4 , cannot. ZrA beads have a high specific surface area(80.84 m2·g−1) and mesoporous structure (15.3 nm, 0.196 cm3·g−1),resulting in a high Langmuir adsorption capacity of 52.5 mg-P/g. Nearly100% of the ZrA adsorbent can be recycled, and HZO mass loss isprevented. Furthermore, the strontium alginate gel framework protectsthe encapsulated HZO from adverse humic acid contamination to en-sure a good phosphorus recovery performance, while naked HZO is sub-ject to severe contamination and, in turn, has a dramatically decreasedadsorption capacity. ZrA beads can be regenerated for at least 5 adsorp-tion/desorption cycles. This work not only provides a novel, simple, andenvironmentally benign solution to realize efficient phosphate recoverybut also implies possible applications of strontium alginate gel beadsencapsulating other functional nanomaterials in numerous otherapplications.

Table 4Cost analysis of ZrA bead raw materials.

Alginate sodium SrCl2 CaCl2 ZrOCl2

Unit price (USD/kg)a 19.2 8.3 3.8 118Mass (kg/kg-ZrA) 0.645 0.43 0.568Unit cost (USD/kg-ZrA) 12.4 3.57 66.7Cost percentage 15% 4.3% 80.7%

a The prices are according to www.casmart.com.cn, 1 USD = 6.7744 CNY.

http://www.casmart.com.cn


Acknowledgments

This research was supported by the National Natural Science Foun-dation of China (21701052), the Natural Science Foundation of Guang-dong Province of China (32217073), the Special Funds for BasicScientific Research Operations of Central Universities of China(21617323), Guangzhou Science and Technology Program(201804010401, China).

Declaration of competing interest

The authors declare no conflicts of interests.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2019.05.296.

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Alkali resistant nanocomposite gel beads as renewable adsorbents for water phosphate recovery1. Introduction2. Experimental2.1. Materials2.2. Synthesis of hydrous zirconium oxide (HZO)2.3. Synthesis of the ZrA beads2.4. Adsorption isotherms of phosphate2.5. Adsorption kinetics2.6. Influence of preloading of humic acid2.7. Regeneration of phosphate-adsorbed gel beads2.8. Characterizations

3. Results and discussion3.1. Alkali resistance of ZrA gel beads using diverse crosslinking ions3.2. Physical structures of the as-synthesized ZrA beads3.3. Phosphate adsorption capacity of the ZrA beads3.4. Adsorption kinetics of ZrA3.5. Advantages of ZrA beads in preventing HZO nanoparticle mass loss3.6. Advantages of ZrA beads against humic acid contamination3.7. Performance of ZrA beads in simulated wastewater phosphate recovery3.8. Regeneration under an intense alkali condition3.9. Economic cost analysis of raw materials for ZrA beads

4. ConclusionAcknowledgmentsDeclaration of competing interestAppendix A. Supplementary dataReferences

science of the total environment - jnu.edu.cn...2.2. synthesis of hydrous zirconium oxide (hzo)...

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